1. Field of the Invention
The present invention relates to an image recording apparatus and, more particularly, to a color image recording apparatus for forming a color image by printing inks of a plurality of colors.
2. Related Background Art
Hitherto, an ink jet recording apparatus for forming an image by spouting ink from nozzles each having a small diameter has been well known. The ink jet recording apparatus is widely used as a color image recording apparatus because a color image can be easily obtained by overlapping printing inks of a plurality of colors.
FIG. 5 shows a scanning method of ink jet heads in the case of obtaining a color image by overlapping the inks of three colors of yellow, magenta, and cyan.
In the diagram, reference numeral 1A, 1B, and 1C denote multi-nozzle heads which are arranged at a distance d from each other. These heads are scanned on a recording sheet 3 at a velocity v in the direction of an arrow 4 while spouting ink from orifices 2.
The head 1A is used for the yellow ink. The head 1B is used for the magenta ink. The head 1C is used for the cyan ink. The inks are printed on the recording sheet 3 in accordance with the order of yellow, magenta, and cyan.
FIG. 6 is a block diagram for the image signal processes by the foregoing ink jet recording apparatus. Input signals 5a to 5c indicative of image densities of three colors of yellow, magenta, and cyan are supplied to a color processing section 6 and subjected to color processes such as masking process and the like. Thereafter, the color processed signals are input to a gradation-correction section 7 and are .gamma. corrected. Among the three-color signals after the correction, the yellow signal is directly sent to a recording head 9A. However, the magenta and cyan signals are first stored into buffers 8A and 8B and thereafter, they are delayed by the times corresponding to a distance d in the scanning direction of the recording head, i.e., by only the time of d/v in the case of the magenta signal and by only the time of 2d/v in the case of the cyan signal. The delayed magenta and cyan signals are sent to heads 9B and 9C. Thus, the inks of the respective colors of yellow, magenta, and cyan are printed on the same position on the recording sheet 3 and a color image is reproduced.
In the gradation-correction section 7, the .gamma. correction is performed in such a manner that the relation of the image density of the printed image for the input image density signal is linear with respect to each color of yellow, magenta, and cyan. The .gamma. characteristics after the correction of yellow, magenta, and cyan become as shown in FIG. 7.
However, these .gamma. characteristics are obtained in the case where the inks of yellow, magenta, and cyan were printed by a single color, respectively. The .gamma. characteristics in the case of two or three colors differ from them.
In the case of the mixed colors, the .gamma. characteristic of each color component depends on the amount of ink which was previously printed.
FIG. 8 shows a change in .gamma. characteristic of magenta to the amount of yellow in which has previously been printed. In FIG. 8, reference numeral 10a denotes a .gamma. characteristics of magenta in the case where the magenta ink was first printed without printing the yellow ink. With an increase in print amount of yellow ink, the .gamma. characteristic of the magenta ink changes as shown in 10b to 10d.
This change is not concerned with the kind of ink. The same tendency is obtained if the printing order is the second or subsequent orders.
It is considered that this phenomenon occurs by the nonlinear mechanism when the ink is absorbed into a sheet. However, the relation between the output image density signal and the color component of the output image becomes nonlinear due to such a phenomenon. There is a drawback such that the color reproduction cannot be sufficiently performed by the linear color correcting processes such as linear masking method and the like. For example in the case of the ordinary linear masking method, when it is assumed that the input yellow, magenta, and cyan signals are set to Y, M, and C, the conversion is performed as follows. EQU Y'=a.sub.11 Y-a.sub.12 M-a.sub.13 C EQU M'=a.sub.21 Y+a.sub.22 M-a.sub.23 C EQU C'=a.sub.31 Y-a.sub.32 M+a.sub.33 C
However, according to this method, the relation of the output signal to the input signal is linear and it is impossible to correct the printer characteristic which changes nonlinearly in accordance with an amount of ink which has previously been printed.
To eliminate the above drawback, a method whereby the nonlinear conversion is performed for the input signal has been proposed. However, this method has a drawback such that a constitution of the apparatus becomes complicated and the cost increases.
As examples of performing the nonlinear conversion to the input signal, there have been proposed the techniques discussed in U.S. Pat. No. 4,614,967, U.S. Pat. No. 4,683,492, U.S. Pat. No. 4,643,563, U.S. Pat. No. 853,768, U.S. Pat. No. 4,631,578 and the like.